What is the distance that separates near-sightedness and far-sightedness?

I think the middle-ground is regular vision? You could have astigmatism if you have trouble seeing near and far.

Stand twenty feet away and see whether you can read large street signs.

Then hold a book at normal length from your face and see whether the letters appear sharp or have blurred edges.

Astigmatism is an irregularly shaped cornea that makes things asymmetrically blurry.

I am both near-sighted and astigmatic. I got my first pair of glasses in 4th grade, my first bifocals when I turned 40. Now I have special glasses for both the computer and playing the piano.

At the library I have a terrible time reading the titles on the bottom shelf. Librarian frowns on me lying down and squinting.

Can someone get me some glasses so I can read the question…...
I work in front of a computer screen all day and sometimes worry if I’m eventually going to end up damaging my eyes.
My wife is short in one eye, long in the other with astigmatism.

Well, I had HORRIBLE eyesight. Like @gailcalled got glasses in fourth grade, and had to change prescriptions every six months. Then, when I was a Senior in HS my folks heard that contact lenses can arrest astigmatism, so they got e some for my graduation present (They were $300 back then) and it did stop it, thank goodness. For me, anything further away than about 12 inches went blurry. So wouldn’t it depend on how bad it is? For others it might be10 feet. Or did I misunderstand the question….

“Librarian frowns on me lying down and squinting.”

Why? As long as you are quiet about it I can’t see the objection.

I would imagine the difference is how far you go away from 20/20, either way.

I started off far-sighted, with my first pair of glasses for reading in the 2nd grade. At 20, I suddenly realized one day standing in line that I couldn’t read a sign I should have been able to read. Got a pair of near-sighted glasses and couldn’t believe the difference, also couldn’t believe that I hadn’t realized I needed glasses for so long. My vision is like 30/20, not too bad, and miraculously hasn’t changed in the 13 years I’ve had glasses.

@Haleth is correct, the terms technically rely on the eyes being in their relaxed state with emmetropia being in the middle

Without getting too technical, near-sighted (myopic) people can never make things far away clear without correction (things at optical infinity; 20+ feet away) but can see things up close without correction.

Far-sighted (hyperopic) people is a weird case because of the ability of the eyes to accommodate. They can see things far away because they can accommodate (essentially make their eyes add additional plus powered lenses by themselves). For their near work, they cannot read things up close COMPARED TO them wearing corrective lenses.

This can be the reason why a lot of kids are misdiagnosed at the school nurse or pediatrician’s office. They’ll be called “perfect” when they read the 20/20 line, but their eyes may be accommodating/straining like crazy to get there.